Research results 01

Synthesis of 2D nanosheets via massive swelling and exfoliation of layered crystals

T. Sasaki , Y. Ebina , R. Ma

We developed 2D oxide and hydroxide nanosheets by inducing enormous swelling of the starting layered materials. The swollen crystals were disintegrated into molecularly thin elementary layers with very high 2D anisotropy. The colloidal nanosheets were applied in “material nanoarchitectonics” to develop hierarchical materials with unique, advanced functionalities

Recently, graphene-related 2D materials known as “beyond graphene” have received increasing attention. Oxide and hydroxide nanosheets are promising candidates because they are expected to exhibit many important properties. We found that platy microcrystals of layered metal oxides undergo more than 100-fold accordion- like expansion in aqueous amine solutions, induced via even permeation of the solution into the interlayer gallery to prop it open^[1]. Interestingly, the degree of swelling is not dependent on amine agents, while the stability of the swollen crystals is. We controlled their delamination to produce unilamellar oxide nanosheets in high yield^[2]. Starting from layered metal oxides synthesized in a specific composition, structure, and size, we obtained high-quality oxide nanosheets, with Ti_0.87O₂, Ca₂Nb₃O₁₀, and Cs₄W₁₁O₃₆ as typical examples.

We also developed synthetic routes to produce hydroxide nanosheets based on transition metal or rare earth elements. One achievement was the synthesis of all-transition metal hydroxide nanosheets of M²⁺_(1-x)M³⁺ x(OH)2, where M = Co, Fe, Ni, Mn. We first synthesized brucite-type layered hydroxides based on divalent metal ions via homogeneous precipitation, which were chemically oxidized into the layered double-hydroxide (LDH) structure. The LDH sample was then delaminated into unilamellar nanosheets by treatment with organic solvents such as formamide.

Through those processes, we produced a range of colloidal suspensions of monodisperse 2D nanosheets, which are useful building modules to fabricate functional materials and nanodevices. Fascinating electronic and optical functionalities were revealed with oxide nanosheets. Superior electrochemical and electrocatalytic performance in transition metal-based hydroxide nanosheets after alternate assembly with graphene was seen^[3].

Main Papers

1. “Unusually stable ~100-fold reversible and instantaneous swelling of inorganic layered materials”, F. Geng, R. Ma, A. Nakamura, K. Akatsuka, Y. Ebina, Y. Yamauchi, N. Miyamoto, Y. Tateyama, T. Sasaki, Nat. Commun. 4 (2013) 1632.
2. “Gigantic swelling of inorganic layered materials: a bridge to molecularly thin two-dimensional nanosheets”, F. Geng, R. Ma, Y. Ebina, Y. Yamauchi, N. Miyamoto, T. Sasaki, J. Am. Chem. Soc. 136 (2014) 5491.
3. “Molecular-scale heteroassembly of redoxable hydroxide nanosheets and conductive graphene into superlattice composites for high performancesupercapacitors”, R. Ma, X. Liu, J. Liang, Y. Bando, T. Sasaki, Adv. Mater. 26 (2014) 4173.